Thermoplastic aromatic polycarbonates having unsaturated terminal groups are known (see German Offenlegungsschriften Nos. 2,746,139 (LeA 18,392), 2,829,256 (LeA 18,847), 2,843,154 (LeA 19,147) and 2,842,004 (LeA 19,148)). The double bonds of these polycarbonates can be utilized for certain reactions (grafting, UV photocrosslinking).
However, there is no controlled molecular weight build-up on heating these polycarbonates, and in fact these products are still stable at temperatures of 280.degree. C. to 300.degree. C.